Piston to valve contact

B N S slammer

B N S slammer

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Aug 15, 2012
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When I pulled my head this morning for a headgasket issue I noticed where some of the intake valves had slightly hit the pistons... I checked the clearance after I put in the 188/220 and I was coming up with roughly .070 of clearance on each intake measurement... Where could I have gone wrong and how could this be? I measured at three points before and after TDC by running the valve down till it hit the piston with a dial indicator on the rocker...
 
Valve float maybe? What spring setup?


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12v?

So you used dumby springs and had the entire top end assembled, torqued down and you rolled the engine over till it was at its closest point and it was .070? Seems to be to much for that cam
 
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You did put the dial indicator on the valve side of the rocker correct?
 
I put CPP #110 springs in before I put compounds on. No this is on a ppump 24v. When I checked the clearance I was plenty past what they consider safe so I figured this wouldnt be an issue. This is a truck with the stock headgasket so the heads never been off. Yes I was on the valve side and ran the valve down with the lash adjustment screw till the valves his the piston.
 
B N S slammer said:
I put CPP #110 springs in before I put compounds on. No this is on a ppump 24v. When I checked the clearance I was plenty past what they consider safe so I figured this wouldnt be an issue. This is a truck with the stock headgasket so the heads never been off. Yes I was on the valve side and ran the valve down with the lash adjustment screw till the valves his the piston.
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Have to put really light springs on so you can push the valve down by hand while slowly rotating the motor to find the tightest spot. If you check in a few spots you may have missed the closest one.
 
Did all int valves contact all the pistons? How close was the exh valves? When I check PTV I check it with dumby springs at ever degree until I find the closet point. The closet point isn't tdc.
 
Is it possible that these springs could have let the valves float slightly at 85-90 psi? Yes all the intakes were showing marks on the pistons. Some you could barely see but you could still tell... I've seen pictures on here and elsewhere where it was lots worse for others... Exhausts were at roughly .085... I checked at .015 .010 and .005 before and after TDC.
 
Double check the key in the cam gear just to rule that out.
 
DISTURBED said:
Double check the key in the cam gear just to rule that out.
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along with this check your crank gear, had one twist on me a few years ago threw my cam timing off by about 2*. If this is the case weld the gear on and redegree your cam
 
Is it a 108 LSA cam? Very common with that one. Zach has a key that sets it back some to gain clearance.
 
cfunk said:
Have to put really light springs on so you can push the valve down by hand while slowly rotating the motor to find the tightest spot. If you check in a few spots you may have missed the closest one.
Click to expand...


its usually this. i have a lot of experience degreeing DOHC gassers and it takes a lot of time to find the crank degree that offers the least piston to valve clearance. you have to let the valvetrain ride the cam with a dummy spring, zero the dial indicator at each degree, push the valve down till contact, and measure the indicator stroke, them move 1 degree and start over.

on gassers it was typically between 7 and 11 degrees of TDC. the intake and exhaust have inverse tight spots. one will be about 9* before TDC and the other cam will be about 9* after TDC. if you adjust the cam gear in relation to the cam core, start over cuz its all changed.
 
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